164 



DISPERSION OF POLLEN BY ANIMALS. 



In the examples above referred to the insects are not forcibly retained in the 

 flowers, for in fine weather the flowers of Gentians, Magnolias and Poppies are as 



wide open as they can be. But there are also cases 

 where insects, after slipping into a floral cavity 

 for shelter, are kept there for a time imprisoned. 

 This remarkable phenomenon is exhibited especially 

 by the Aroideae and Aristolochiacese. In many 

 Aroidese (AruTn, Bracunculus, Helicodiceros, &c.), 

 of which Arwm conocephaloides (flg. 241) may be 

 taken as a type, the ensheathing spathe widens out 

 above, whilst below the middle there is a decided 

 constriction, and the lowest part expands into a 

 barrel-shaped receptacle. The temperature inside the 

 cavity is always considerably above that of the en- 

 vironment, and ranges not infrequently from 30° to 

 36° C; in the spathes of the Italian Arum (Arum 

 Italicwm) a temperature of 44° C. has even been 

 recorded (see vol. i. p. 501). All these Aroideee have 

 an ofiensive odour of putrefaction, and by this very 

 property attract a number of animals which live on 

 dead bodies and other decaying matter. These crea- 

 tures settle on the projecting end of the spadix and 

 ■climb down it into the barrel-shaped cavity, where 

 they find a warm habitation and in addition a supply 

 of food in the thin- walled succulent cells lining the 

 interior. At the part where the spathe is con- 

 stricted the spadix is encompassed by a ring of stiff 

 bristles, which form a contrivance like a lobster-trap. 

 The points of most of the bristles are curved down- 

 wards, so as to allow the insects to climb down into 

 the chamber but prevent their egress. It is not till 

 some days later that the bristles become limp, the 

 constriction in the spathe is loosened and expanded, 

 and the captives are able to leave their temporary 

 prison, and by that time the pollen has been liber- 

 ated from the anthers and covers that region of the 

 spadix which bears the male flowers ; it is thus 

 impossible for insects to climb up the spadix without 

 first becoming loaded with the pollen lying in their 

 way, and they afterwards carry it to other younger 

 flowers. In Arwm conocephaloides (fig. 241) there 

 are two rings of bristles, one above the other. 

 The upper hairs relax later than the lower ones, and when, after the latter have 



Fig. 241. — Arum conocephaloides, with the 

 front wall of the spathe removed. On 

 the lowest part of the spadix are the 

 female flowers, above them the first ring 

 of bristles, next the male flowers, and 

 then a second ring of bristles. At the 

 bottom of the cavity are a number of 

 midges belonging to the genus Cerato- 

 pogon whose escape is prevented by 

 the stiff deflexed points of the lower 

 ring of bristles. 



